1. Field of the Invention
This invention relates to a failure-detecting device for air-fuel ratio sensors (e.g. O.sub.2 sensors) provided in the exhaust passages of internal combustion engines at locations downstream of three-way catalysts.
2. Prior Art
A method of detecting deterioration/failure of air-fuel ratio sensors for internal combustion engines has been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-11840, in which a correction amount, which corrects the fuel injection amounts of fuel injection valves, is controlled by using a forced duty ratio waveform signal having a constant repetition period T and a constant amplitude A as shown at (a) of FIG. 1, when the engine is in a steady operating condition, such that the air-fuel ratio of an air-fuel mixture in the intake passage of the engine is forcedly perturbed, and the duty ratio of an output from an O.sub.2 sensor provided upstream of a three-way catalyst in the exhaust passage, which depends on the perturbed air-fuel ratio, is detected, to thereby detect a degree of deterioration of the O.sub.2 sensor based on the detected duty ratio. Examples of results of detection of the O.sub.2 sensor output duty ratio are shown at (b), (c), and (d) of FIG. 1. (b) of the figure shows a case in which the O.sub.2 sensor output is deviated toward the richer side due to deterioration of the sensor, (c) a case in which the O.sub.2 sensor output is normal, and (d) a case in which the O.sub.2 sensor output is deviated toward the leaner side due to deterioration of the sensor. In (b), (c), and (d) of the figure, VOX designates output voltage of the O.sub.2 sensor, VR reference voltage (e.g. 0.45 V), TR and TL periods over which the output voltage VOX is higher than the reference voltage VR (i.e. the air-fuel ratio is on the rich side) and lower than same (i.e. the air-fuel ratio is on the lean side), respectively. When the duty ratio (TR/(TR+TL)) of the output voltage VOX is within a normal range as shown in (c) of FIG. 1, it is judged that the O.sub.2 sensor is normal, whereas when the output of the O.sub.2 sensor is deviated toward the richer side or the leaner side so that the duty ratio is excessively high or low, it is judged that the O.sub.2 sensor is deteriorated or faulty.
According to the above method of detecting deterioration/failure of the O.sub.2 sensor, the deterioration/failure of the O.sub.2 sensor is determined based on the duty ratio of output voltage from the O.sub.2 sensor, i.e. the ratio of a time period over which the sensor output falls in the richer side to the repetition period. However, in the case where the O.sub.2 sensor is provided downstream of the three-way catalyst, the output of the O.sub.2 sensor does not vary to a large degree due to oxygen storage effect of the three-way catalyst (e.g. it does not vary in such a large range as from 0.1 V to 0.9 V, for example), so that the deterioration/failure of the O.sub.2 sensor cannot be determined correctly by the above method based on the duty ratio of the O.sub.2 sensor output.